The present invention relates to a sintered body of silicon nitride (Si.sub.3 N.sub.4) with a good high-temperature strength. The present invention also relates to a composite sintered body of silicon carbide (SiC) and silicon nitride, particularly, an HIP composite sintered body with a good high-temperature strength and a high density, and a method for producing the composite sintered body.
A sintered body of silicon nitride has been expected to be used as a material for engine parts of automobiles, rotors, gas turbine parts such as shrouds and nozzles, sliding members, tools, etc. because of its high-temperature strength, high hardness and high wear resistance superior to those of the ceramics of other types.
Since silicon nitride powder is hardly sintered, a densified sintered body is usually produced by using a sintering aid such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 and MgO. However, a glass phase having a low melting point likely remains in the grain boundaries of silicon nitride when such oxides are used in combination, thereby resulting in the deterioration of high-temperature strength, in particular, strength at 1200.degree. C. or higher.
Several proposals to improve high-temperature strength of sintered body have been made.
JP-A-63-100066 discloses precipitating crystals of XSiO.sub.2 N(X is a IIIa-Group element of the Periodic Table) in the grain boundary phase of silicon nitride by cooling a sintered product in a slow cooling rate. However, it is difficult to crystallize the inner portions of a sintered product when producing thick or bulky bodies, and a long cooling time is required to reduce the productivity of sintered products.
JP-A-4-46062 discloses sialon solid solutions formed from Si.sub.3 N.sub.4 and the oxide constituting the grain boundary phase. However, a sintered body thus produced is still insufficient in oxidation resistance and shows a significant reduction of strength at 1300.degree. C. or higher.
JP-A-4-280871 discloses the combined use of an oxide of a rare earth element and an oxide of a IVa-Group element such as HfO.sub.2, etc. as the sintering aid. Although a sintered body thus produced is improved in strength at 1300.degree. C. or higher to some extent, it is insufficient in fracture toughness and creep characteristics.
Because of an excellent high-temperature strength, high oxidation resistance, high wear resistance, high hardness, etc., a silicon carbide ceramic sintered body is also expected to be useful in the same applications as in the case of the silicon nitride ceramic sintered body described above. Recently, various attempts have been made to provide, in place of such monolithic sintered bodies, a composite sintered body of silicon nitride and silicon carbide having advantages of both the ingredients. Such a composite sintered body includes a silicon nitride-silicon carbide composite sintered body having nano-composite structures in which fine silicon carbide particles are dispersed in silicon nitride particles.
JP-A-2-160669 discloses the production of a silicon nitride-silicon carbide composite sintered body from an amorphous silicon nitride-silicon carbide composite powder obtained by a vapor phase reaction method. The composite sintered body has a fine structure in which silicon carbide particles having an average particle size of 1 .mu.m or less are present along grain boundaries and silicon carbide particles as small as several nanometers to several hundreds of nanometers are dispersed in silicon nitride particles. However, since the amorphous silicon nitride-silicon carbide composite powder is decomposed during sintering, it is not easily subjected to a liquid-phase sintering. Also, since the amorphous silicon nitride-silicon carbide composite powder is extremely bulky, it is not easily molded. Accordingly, the amorphous silicon nitride-silicon carbide composite powder should practically be sintered by hot pressing, and therefore, the method of this document is not applicable to producing a product of complicated shape.
JP-A-2-255572 discloses a silicon nitride-silicon carbide composite sintered body formed by sintering a powdery mixture of silicon nitride, silicon carbide, aluminum nitride and yttria. Although this composite sintered body has .beta.-silicon nitride phase, .beta.-silicon carbide phase and .alpha.-sialon phase and shows a high strength at ordinary temperature, it is still insufficient in high-temperature strength, in particular at 1300.degree. C. or higher. With respect to sintering aid, this document discloses only yttria and considers nothing as to the rare earth oxide sintering aid other than yttria which cannot provide a sintered body with a sufficient high-temperature strength as indicated above.
JP-A-3-205363 discloses a silicon nitride-silicon carbide composite sintered body formed by sintering a powder mixture of silicon nitride, silicon carbide and a compound of a rare earth element. Since the grain boundary phase of the silicon nitride particles consists essentially of a crystalline phase, the composite sintered body shows a high-temperature strength comparable to the strength at room temperature. However, the densification of the sintered body may be prevented by a slight temperature change during the sintering step due to the absence of aluminum nitride. Therefore, silicon carbide is added in a small amount to ensure a sufficient densification. However, this minimizes the effect of adding silicon carbide to silicon nitride.